Typical integrated circuit (IC) audio amplifiers deliver only a few watts of power but have proven useful in many applications. However, there is a need for high power devices that can deliver 20 watts or more for high fidelity applications. The lower power devices are not limited by the basic characteristics of the semiconductor used in fabrication but rather by other constraints that enter into IC design.
In high power IC audio amplifiers the output transistors dominate the IC chip area. Typically the back side of the chip is soldered or otherwise secured in close thermal contact with a metallic heat sink. Heat sinks such as those provided by the industry standard TO-3 or TO-220 packages have proven to be useful. Even with this construction the thermal resistance of the silicon chip is such that temperature gradients exist across the face of the power transistors and hence across the IC chip. Additionally the temperature of the material containing the output transistors rises substantially with applied signals and power. These temperature rises and gradients can not only react on the output transistors themselves, they can react adversely to other circuitry contained on the IC chip.
My copending application Ser. No. 056,489 filed July 11, 1979, is titled AMPLIFIER OUTPUT STAGE DISTORTION REDUCTION and shows some circuitry associated with a class B output stage in an IC.